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aberration
in optics, an imperfect focus caused when a mirror or lens fails to bring light to a sharp focus
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absolute magnitude
the apparent brightness an object would have if it were 10 parsecs (32.6 light-years) from Earth
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absolute zero
the coldest theoretical temperature, equal to 0 kelvin (-459.67° F or -273.15° C)
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absorption lines
dark lines in a spectrum caused by the absorption of light by atoms or molecules in a star or planet’s atmosphere
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accretion disk
a disk surrounding a black hole or star in which matter gravitationally falls onto the central object
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achromatic lens
a two-element lens, or doublet, that significantly reduces chromatic aberration
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active galactic nuclei
the exceptionally bright cores of some galaxies, thought to be fueled by matter falling into supermassive black holes
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active galaxy
a galaxy emitting unusually large amounts of energy from a compact central source
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active optics
The techniques by which corrections are made to the shape of a large mirror or radio dish to adjust for minute distortions in its shape. These variations arise as a telescope is subjected to forces such as gravity and temperature changes.
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adaptive optics
a system of telescopes, computers, and deformable mirrors used to compensate for atmospheric blurring
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albedo
the percentage of light that an object reflects
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altazimuth mount
a mount that enables a telescope to move freely both vertically (in altitude) and horizontally (in azimuth)
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altitude
1. the height above sea level
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anaglyph
a stereoscopic, composite image in which the right component (usually red in color) is superimposed on the left component (usually blue) to
produce a three dimensional effect when viewed through correspondingly colored filters
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angular size
the apparent width of an object as seen by an observer, usually expressed in degrees, arcminutes, or arcseconds
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anisotropies
differences in physical properties depending on direction
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anisotropy
the variation of a physical property depending on direction
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annular eclipse
a solar eclipse in which the moon does not fully cover the sun’s disk, allowing observers to see a thin ring of sunlight
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antimatter
matter consisting of particles that have the same mass and properties as their matter counterparts but opposite electrical charges
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anti-tail
The name given to a comet's tail when it points toward the sun. This rare event typically occurs when Earth crosses the plane of the comet's orbit and the comet is relatively close to the sun.
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aperture
the diameter of a telescope’s primary lens or mirror; the larger the aperture, the greater the telescope’s light-gathering power
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aphelion
the point farthest from the sun in an object’s orbit
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apochromatic lens
a lens with three or more elements that reduces chromatic aberration even more than an achromatic lens
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apogee
the point in a satellite’s orbit when it is farthest from Earth
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apparent field of view
the angular diameter of the circle of light that the eye sees through an eyepiece
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apparent magnitude
the measure of the brightness of an object as seen from Earth
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apparition
the period of time during which a particular celestial object can be seen
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archeoastronomy
the study of physical artifacts with astronomical connections
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arcminute
a unit of angular size equal to 1/60 of a degree; abbreviated by '
Arcminutes are used to measure of the separation between two sky objects or the angular size of an object.
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arcsecond
a unit of angular size equal to 1/3,600 of a degree (or 1/60 of an arcminute); abbreviated by "
Arcseconds are used to measure of the separation between two sky objects or the angular size of an object.
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asterism
a small grouping of stars in the night sky
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asteroid
a small, rocky body that orbits a star
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asteroid belt
the zone in which most asteroids orbit the sun, located between the orbits of Mars and Jupiter
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astrometry
the study of the positions and motions of celestial objects
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astronomical unit
the average distance from Earth to the sun, equal to about 93,000,000 miles (150,000,000 km)
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astronomical units
a measurement used by astronomers within the solar system; one astronomical unit (AU) is the average distance between Earth and the sun (about 93,000,000 miles or 150,000,000 kilometers)
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astronomy
the branch of science concerned with objects beyond Earth
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astrophotography
the photography of astronomical objects
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astrophysics
the branch of astronomy that deals with the physical characteristics of celestial objects
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atmosphere
a gaseous envelope surrounding a moon, planet, or star
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atom
the fundamental unit of matter; can consist of protons, neutrons, and electrons
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atomic nucleus
the central region of an atom; can consist of protons and neutrons
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attitude
the orientation of a spacecraft relative to the direction of its motion
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aurora
the emission of light when charged particles from the solar wind slam into and excite atoms and molecules in a planet’s upper atmosphere
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aurora australis
the southern lights; see definition for aurora
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aurora borealis
the northern lights; see definition for aurora
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aurorae
the plural of aurora (the emission of light when charged particles from the solar wind slam into and excite atoms and molecules in a planet’s upper atmosphere)
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autoguider
a CCD camera used to automatically guide a telescope during long-exposure photography
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autumnal equinox
the time of year around September 23 when the sun crosses the celestial equator heading south
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averted vision
a technique that uses the more light-sensitive rods in the eye to better see a faint object by looking at it indirectly
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axis
a straight line about which an object rotates
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azimuth
the angle along the horizon measured eastward from due north to the point on the horizon directly below an object
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